Form Designerで「Collection is Read-Only」エラーメッセージが表示される


解説

Form Designerでフォームに最初のフィールドを追加するとき、ブラウザに「Collection is Read-Only」というエラー メッセージが表示されることがあります。

例外とそのスタック トレースの例は次のとおりです：

Collection is read-only.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
Exception Details: System.NotSupportedException: Collection is read-only.
Source Error: 
An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
Stack Trace: 
[NotSupportedException: Collection is read-only.]
   System.Collections.Specialized.NameValueCollection.Add(String name, String value) 
   Sitecore.Forms.Shell.UI.Controls.FormBuilder.AddToSetNewField() 
   Sitecore.Forms.Shell.UI.FormDesigner.AddNewField() 
[TargetInvocationException: Exception has been thrown by the target of an invocation.]
   System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor) 
   System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments) 
   System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) 
   Sitecore.Shell.Framework.Commands.MethodCommandProcessor.Invoke(PipelineArgs args) 
   Sitecore.Nexus.Pipelines.NexusPipelineApi.Resume(PipelineArgs args, Pipeline pipeline) 
   Sitecore.Pipelines.Pipeline.Start(PipelineArgs args, Boolean atomic) 
   Sitecore.Shell.Framework.Commands.MethodCommand.Execute(CommandContext context) 
   Sitecore.Web.UI.Sheer.ClientPage.Dispatch(String command) 
   Sitecore.Web.UI.Sheer.ClientPage.RaiseEvent() 
   Sitecore.Web.UI.Sheer.ClientPage.OnPreRender(EventArgs e) 
   System.Web.UI.Control.PreRenderRecursiveInternal() 
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

この問題はWFFM 8.2.0～8.2.3のバージョンで発生します。

解決策

この問題を解決するには、以下のオプションのいずれかを検討してください：